The present invention relates to a prosthetic disc nucleus. More particularly, it relates to a hydrophilic prosthetic spinal disc nucleus packaged and provided to surgeons in a partially hydrated state.
The vertebral spine is the axis of the skeleton upon which all of the body parts xe2x80x9changxe2x80x9d. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar segments sit upon a sacrum, which then attaches to a pelvis, in turn supported by hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints, but allow known degrees of flexion, extension, lateral bending and axial rotation.
The typical vertebra has a thick interior bone mass called the vertebral body, and a neural (vertebral) arch that arises from a posterior surface of the vertebral body. Each narrow arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch that extends posteriorly and acts to protect a posterior side of the spinal cord is known as the lamina. Projecting from the posterior region of the neural arch is a spinous process. The central portions of adjacent vertebrae are each supported by an intervertebral disc.
The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulposus (xe2x80x9cnucleusxe2x80x9d), the anulus fibrosus (xe2x80x9canulusxe2x80x9d), and two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus serve to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring that binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the anulus consist of 15 to 20 overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 30-degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The anulus and opposing end plates maintain a relative position of the nucleus in what can be defined as a nucleus cavity. The healthy nucleus is largely a gel-like substance having a high water content, and similar to air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae with bending, lifting, etc.
The nucleus and the inner portion of the anulus have no direct blood supply. In fact, the principal nutritional source for the central disc arises from circulation within the opposing vertebral bodies. Microscopic, villous-like fingerlings of the nuclear and anular tissue penetrate the vertebral end plates and allow fluids to pass from the blood across the cell membrane of the fingerlings and then inward to the nuclear tissue. These fluids are primarily body water and the smallest molecular weight nutrients and electrolytes.
The natural physiology of the nucleus promotes these fluids being brought into, and released from, the nucleus by cyclic loading. When fluid is forced out of the nucleus, it passes again through the end plates and then back into the richly vascular vertebral bodies. The cyclic loading amounts to daily variations in applied pressure on the vertebral column (e.g., body weight and muscle pull) causing the nucleus to expel fluids, followed by periods of relaxation and rest, resulting in fluid absorption or swelling by the nucleus. Thus, the nucleus changes volume under loaded and non-loaded conditions. Further, the resulting tightening and loosening effect on the anulus stimulates the normal anulus collagen fibers to remain healthy or to regenerate when torn, a process found in all normal ligaments related to body joints. Notably, the ability of the nucleus to release and imbibe fluids allows the spine to alter its height and flexibility through periods of loading or relaxation. Normal loading cycling is thus an effective nucleus and inner anulus tissue fluid pump, not only bringing in fresh nutrients, but perhaps more importantly, removing the accumulated, potentially autotoxic by-products of metabolism.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anular confines. The mass of a herniated or xe2x80x9cslippedxe2x80x9d nucleus can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases, causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae are surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent the fused segment as they compensate for the lack of motion, perhaps leading to premature degeneration of those adjacent discs. A more desirable solution entails replacing in part or as a whole the damaged nucleus with a suitable prosthesis having the ability to complement the normal height and motion of the disc while stimulating the natural disc physiology.
The first prostheses embodied a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetic discs were designed to replace the entire intervertebral disc space, and were large and rigid. Beyond the questionable efficacy of those devices were the inherent difficulties encountered during implantation. Due to their size and inflexibility, these first generation devices required an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve rootlets during posterior implantation, could not be avoided. Recently, smaller and more flexible prosthetic nucleus devices have been developed. With the reduction in prosthesis size, the ability to work around the spinal cord and nerve rootlets during posterior implantation has become possible.
Generally speaking, these reduced sized prostheses are intended to serve as a replacement for the natural nucleus. In other words, the anulus and end plates remain intact, and the prosthesis is implanted into the nucleus cavity through an opening formed in the anulus. To minimize damage or stress on the anulus during implantation, the prosthetic nucleus will preferably expand from a relatively small pre-implant size to a relatively larger post-implant size. In this regard, hydrogel materials have been identified as being highly applicable. Generally speaking, hydrogel materials have a strong affinity for water, and expand upon hydration. With this in mind, a hydrogel-based prosthetic nucleus can be implanted in a relatively small, dehydrated state. Once in contact with the fluids found in the intervertebral disc space the hydrogel will hydrate. With hydration, the hydrogel-based prosthetic nucleus will grow or expand, forcing apart the adjacent vertebrae. When fully hydrated, then, the hydrogel-based prosthetic nucleus has properties highly similar to a natural nucleus, restoring and maintaining the height of a damaged disc space, and tightening the anulus.
Several different potential hydrogel-based prosthetic nucleus devices are described, for example, in Ray et al., U.S. Pat. No. 5,647,295 and Bao et al., U.S. Pat. No. 5,047,055, the teachings of which are incorporated herein by reference. Regardless of exact design, the hydrogel-based prosthesis is dehydrated prior to implant, rendering the device as small as possible. Following implant, the hydrogel material will slowly hydrate to a final hydration level, normally over the course of two or three days. Because the time for hydration is relatively lengthy, the possibility of prosthesis migration or explant back through the anulus opening may arise. In other words, in the dehydrated state, the hydrogel-based prosthetic nucleus has, in theory, a height and width slightly smaller than a height and width of the anulus opening. Because the hydrogel material does not immediately hydrate, and therefore expand, the outer dimensions of the prosthesis continue to correspond with the dimensions of the anulus opening. Therefore, the anulus cannot readily prevent the hydrogel-based prosthetic nucleus from migrating back through the anulus opening. Even if this opening is closed via sutures following implant, various forces acting upon the spine have the potential to xe2x80x9cpushxe2x80x9d the prosthesis back through the anulus opening. In this regard, the hydrogel material is extremely hard in the dehydrated state, thereby increasing the likelihood of spontaneous explant. That is to say, the absence of device conformability promotes sliding of the prosthesis within the nucleus cavity with the placement of a load and/or opposing movement of the end plates.
Additionally, it is often times difficult to implant a properly sized hydrogel-based prosthesis. In theory, a surgeon will evaluate the disc space and select a correspondingly sized prosthetic device. Several factors may impede the surgeon""s ability to implant the so-selected device. First, the implant environment is highly confined, making access to, and maneuvering within, the disc space exceedingly difficult. Also, while the hydrogel-based prosthesis is dehydrated prior to implant, an absolute limit or minimum dehydration size/volume exists. Thus, for example, the Bao device is shown as being extremely small in the dehydrated state, expanding to fill the entire nucleus cavity with hydration. In practice, current hydrogel technology does not allow for such a drastic change (e.g., on the order of 10xc3x97) in volume. Instead, a hydrogel-based prosthesis can only experience a maximum increase in volume (from the dehydrated state to a fully hydrated state) on the order of 2xc3x97. As a result, although the disc space size may be approximated accurately, the corresponding prosthesis device may be too large (in a dehydrated state) to be implanted. Additionally, while the natural nucleus material is desirably completely removed, this is nearly impossible to accomplish, thereby decreasing the implant space. Taken in combination, the above-factors may force the surgeon to instead implant a prosthetic device that is less than optimally sized. Unfortunately, the reduced-sized prosthesis is likely more susceptible to unwanted migration, and may not provide proper discal support.
Degenerated, painfully disabling intra-spinal discs are a major economic and social problem for patients, their families, employers and the public at large. Any significant means to correct these conditions without further destruction or fusion of the disc may therefore serve an important role. Other means to replace the function of a degenerated disc have major problems such as complex surgical procedures, unproven efficacy, placing unnecessary and possibly destructive forces on an already damaged anulus, etc. Further, unexpected migration and explant of the prosthesis, especially a hydrogel-based prosthetic nucleus, from the disc space following implant, while uncommon, may be a potential concern. Therefore, a need exists for a prosthetic spinal disc nucleus implantable in a form having improved conformability and a reduced potential for explant.
One aspect of the present invention relates to a packaged prosthetic disc nucleus. The packaged prosthetic disc nucleus includes a prosthetic disc nucleus and a retainer. The prosthetic disc nucleus is sized for implantation within a nucleus cavity and includes a hydrogel core configured to hydrate from a dehydrated state to a final hydrated state, such as following implant. The retainer selectively contains the prosthetic disc nucleus. Further, upon contact with a hydration liquid, the retainer is configured to allow the hydrogel core to hydrate from the dehydrated state, but prevents the hydrogel core from hydrating to the final hydrated state. For example, in one preferred embodiment, the packaged prosthetic disc nucleus further includes an outer enclosure, such as a pouch, surrounding the retainer and a hydration liquid contained within the enclosure for hydrating the hydrogel core. Regardless of the hydration liquid source, the prosthetic disc nucleus is constrained by the retainer to a partially hydrated state. More particularly, the retainer limits volumetric expansion of the hydrogel core, thereby constraining hydration to the partially hydrated state. During use, the prosthetic disc nucleus is removed from the retainer and preferably implanted within a nucleus cavity in the partially hydrated state. The hydrogel core then hydrates to the final hydrated state by imbibing fluids from within the nucleus cavity. Because the hydrogel core is partially hydrated, the prosthetic disc nucleus has improved cnoformability and will reach the final hydrated state more quickly than a similar prosthetic disc nucleus implanted in a dehydrated state.
Another aspect of the present invention relates to a packaged prosthetic disc nucleus. The packaged prosthetic disc nucleus is sized for implantation within a nucleus cavity and includes a prosthetic disc nucleus and a retainer. The prosthetic disc nucleus includes a hydrogel core configured to hydrate and expand from a dehydrated height to a final hydration height. The retainer selectively contains the prosthetic disc nucleus. When placed in contact with a hydration liquid, the retainer allows the hydrogel core to hydrate and expand from the dehydrated height, but prevents the hydrogel core from attaining the final hydration height. In one preferred embodiment, for example, the packaged device farther includes an outer pouch containing the retainer and a supply of hydration liquid in contact with the prosthetic nucleus. Regardless, with hydration liquid interaction, the retainer is configured to constrain the hydrogel core to a partial hydration height that is less than the final hydration height. During use, the prosthetic disc nucleus, at the partial hydration height, is removed from the retainer and implanted into the nucleus cavity. Because the partial hydration height is less than the final hydration height, a size of a requisite opening in the anulus can be reduced. Further, in accordance with one preferred embodiment, the retainer dictates that the partial hydration height is less than a natural or unloaded dehydrated height, thereby promoting selection and implantation of a properly sized prosthesis.
Yet another aspect of the present invention provides a method of packaging a prosthetic disc nucleus including a hydrogel core. The hydrogel core is configured to hydrate from a dehydrated level for subsequent implantation within the nucleus cavity where the hydrogel hydrates to a final hydration level. The method includes dehydrating the hydrogel core. A retainer is provided forming a cavity sized to selectively contain the prosthetic disc nucleus. The prosthetic disc nucleus is placed within the cavity. The combination prosthetic disc nucleus/retainer is then allowed unimpeded contact with a hydration liquid such that the hydrogel core hydrates. In one preferred embodiment, the combination retainer/prosthetic disc nucleus is placed within an outer enclosure. The outer enclosure is then at least partially filled with the liquid for hydrating the hydrogel core. Finally, the outer enclosure is sealed for subsequent delivery to a surgeon. Regardless of the source, the retainer constrains expansion and therefore hydration of the hydrogel core to a partial hydration level that is less than the final hydration level.